The present invention relates to light polarizing glass articles, a process for preparation thereof and an optical isolator using such glass articles. The light polarizing glass articles of this invention make it possible to reduce in size the optical isolators utilized in optical communication techniques using semiconductor lasers and optical fibers.
Optical isolators are used to cut off the backward light caused by reflection to improve the S/N ratio in optical fiber communication techniques utilizing semiconductor lasers at a wavelength of 1.31 .mu.m or 1.55 .mu.m as a light source. The optical isolators basically comprise a Faraday rotational element, two polarizers and a magnet, and it is necessary to reduce the size of each element to miniaturize the isolators. It is difficult, however, to miniaturize polarizers while maintaining its extinction ratio and environment resistance.
Polarizing glasses in which metal particles having large aspect ratios are aligned seemed to satisfy the above requirements. U.S. Pat. No. 3,954,485 discloses polarizing glasses and glass articles containing copper. According to the patent, the articles are prepared as follows. An aluminoborosilicate based glass containing halogen elements and copper oxide and/or cadmium oxide (in one example, copper oxide and cadmium oxide) was heated to a temperature of from 500 to 900.degree. C. to cause the development therein of second phase droplets having a size of 100-1000 .ANG., which is a boron rich glassy phase containing copper and cadmium halides. The resulting glass article was elongated by a factor of about 50, at which time the second phase droplets therein achieve aspect ratios in the range of 2:1 to 5:1 as the temperature of the glass article is adjusted such that the glass exhibits a viscosity in the range of 1.times.10.sup.7 to 1.times.10.sup.9 poises. The glass article was then cooled to a temperature below the annealing point thereof to obtain the polarizing glass article.
Since the glass articles are provided for the purpose of eye-glasses, the size of the second phase is less than 1000.ANG. (100 nm) in order to keep the articles transparent in the visible light range. The articles also exhibit photochromic properties. Further, the glass article exhibits polarizing properties in the darkened state (the colored state caused by irradiation of light) as obtained by irradiation of light with a short wavelength (e.g. 0.3-0.45 .mu.m). However, in the darkened state, the extinction ratio of the glass articles is about 10:1 (10 dB) which is smaller than the extinction ratio (30 dB) required for an optical isolator used in optical communication.
The reasons why the extinction ratio of the glass articles is smaller are described in U.S. Pat. No. 3,954,485 and a report in Journal of Non-crystalline Solid, Vol. 33, pp 383-390 (1979). That is, the second phase droplets in the glass containing copper, cadmium and chlorine have a structure comprising a spherical droplet which ranges from 100 to 1000 .ANG. in diameter and is a borate rich glassy phase. The glasses also comprise a smaller particle structures, ranging from 20 to 50 .ANG. in diameter, which consist of copper chloride and cadmium chloride and provide photochromic properties to the glass. The spherical second phase droplet is elongated by redrawing of the glass, while the smaller particle structure is not elongated. This is because elongation of the smaller particles requires more stress and is hard to occur since the particle size of the smaller particle is too small.
The reasons why the glass exhibits weak polarizing properties are that the smaller particles consisting of copper and cadmium chlorides are aligned anisotropically on the surface of the elongated second phase particles.
It is impossible to change the glass to the darkened state by irradiation of the infrared light with a wavelength of 1.3-1.55 .mu.m which is usually used in optical communication techniques, and therefore, the extinction ratio of the glass obtained by the irradiation of the infrared light is extremely low. Thus it is impossible to apply the copper containing glass disclosed in U.S. Pat. No. 3,954,485 to polarizers used in optical isolators for the optical communication techniques.
U.S. Pat. No. 4,479,819 discloses a preparation of polarizing glass articles wherein silver halide particles are generated in a glass body, this glass body is elongated such that the particles are elongated to an aspect ratio of at least 5:1 and the elongated glass body is exposed to a reducing environment to reduce the silver halide particles in the glass to metallic silver.
Silver halide particles with relatively small size is required to obtain anisotropic metallic silver particles exhibiting polarizing properties. However, it is difficult to form anisotropic silver halide particles with small size by elongation of a glass body containing silver halide particles. Further, unreduced silver halide particles remained in the glass exhibit large scattering loss, and a polarizing glass with the scattering loss is not preferred.
U.S. Pat. No. 3,653,741 discloses articles comprising a matrix having dispersed substantially uniformly therethrough a plurality of electro-optically responsive dipole particles. Examples of the dipole particles include copper particles and glass is exemplified as the matrix. It is disclosed that continuous drawing methods may be employed using a glass melt containing dipoles, and the drawing and rolling of the glass will cause the orientation of the dipolar particles to produce polarized glass.
It is necessary that dipole Cu particles such as Cu whiskers are uniformly dispersed in glass melt in order to produce polarizing glasses containing dipole Cu particles. However, if a temperature of glass melt is too high or if dispersion of the whiskers is required a relatively long term, polarizing glasses are not obtained because Cu whiskers melt to form Cu spheres or are oxidized to form Cu ions. Thus glasses with relatively low Tg and melting points should be used to form polarizing glasses by the method described in U.S. Pat. No. 3,653,741.
On the other hand, when assembling optical isolators, polarizing glasses are partially melted and adhered by heating at a high temperature, for example, at about 400-450.degree. C. Thus polarizing glass used for the optical isolators should have heat resistance. However, base glasses for the polarizing glasses made by the method of the above U.S. Patent are limited to glasses with low Tg and melting point, but such polarizing glasses are not heat resistant.
An object of the invention is to provide a polarizing glass article containing metallic copper, which exhibits good heat resistance and a high extinction ratio in the infrared region used in optical communication techniques, and a process for preparation thereof. Another object of the invention is to provide an optical isolator useful for optical communication using the polarizing glass article which is a miniature polarizer.